Process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution

ABSTRACT

The present invention relates to a process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution comprising less than 0.3% by weight of persulfate and at least 0.05% by weight of azo initiator and thermal aftertreatment of the formed polymer particles at less than 100° C. in a fluidized bed for 60 to 300 minutes.

The present invention relates to a process for producing water-absorbentpolymer particles by polymerizing droplets of a monomer solutioncomprising less than 0.3% by weight of persulfate and at least 0.05% byweight of azo initiator and thermal aftertreatment of the formed polymerparticles at less than 100° C. in a fluidized bed for 60 to 300 minutes.

The preparation of water-absorbent polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, on pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbent polymerparticles are used to produce diapers, tampons, sanitary napkins andother hygiene articles, but also as water-retaining agents in marketgardening. Water-absorbent polymer particles are also referred to as“superabsorbent polymers” or “superabsorbents”.

The preparation of water-absorbent polymer particles by polymerizingdroplets of a monomer solution is described, for example, in EP 0 348180 A1, WO 96/40427 A1, U.S. Pat. No. 5,269,980, WO 2008/009580 A1, WO2008/052971 A1, WO2011/026876 A1, and WO 2011/117263 A1.

Polymerization of monomer solution droplets in a gas phase surroundingthe droplets (“dropletization polymerization”) affords roundwater-absorbent polymer particles of high mean sphericity (mSPHT). Themean sphericity is a measure of the roundness of the polymer particlesand can be determined, for example, with the Camsizer® image analysissystem (Retsch Technology GmbH; Haan; Germany).

It was an object of the present invention to provide water-absorbentpolymer particles having improved properties, i.e. water-absorbentpolymer particles having a high bulk density, a high centrifugeretention capacity (CRC), a high absorption under a load of 21.0 g/cm²(AUL), a low level of residual monomers and an improved whiteness.

The object is achieved by a process for producing water-absorbentpolymer by polymerizing droplets of a monomer solution comprising lessthan 0.3% by weight of persulfate and at least 0.05% by weight of azoinitiator in a surrounding heated gas phase in a reaction zone andthermal posttreatment in a fluidized bed, wherein the temperature of thegas leaving the reaction zone is less than 150° C., the temperature ofthe water-absorbent polymer particles during the thermal posttreatmentis less than 100° C. and the residence time of the water-absorbentpolymer particles in the fluidized bed is from 60 to 300 minutes.

The present invention is based on the finding that the temperature thewater-absorbent polymer particles in the fluidized bed has a strongimpact on the bulk density, the level of residual monomers and theabsorption under a load of 21.0 g/cm² (AUL). On increasing thetemperature there is a significant decrease of these values. For gettingwater-absorbent polymer particles having a high bulk density, a lowlevel of residual monomers and a high absorption under a load of 21.0g/cm² (AUL) it is advantageous reducing the temperature and prolongingthe residence time in the internal fluidized bed. The reduction of thelevel of residual monomers can be accelerated by increasing the steamcontent of the gas to be used in the internal fluidized bed.

The present invention is further based on the finding that persulfatesare necessary for a low level of residual monomers. Persulfates are alsothe reason for discoloration of the water-absorbent polymer particles.So, only small amounts of persulfates shall be used.

The present invention is further based on the finding that azoinitiators have a strong impact on the centrifuge retention capacity(CRC). Persulfates do not have such impact.

The result of the specific conditions according to the process of thepresent invention are water-absorbent polymer particles having a highbulk density, a high centrifuge retention capacity (CRC), a highabsorption under a load of 21.0 g/cm² (AUL), a low level of residualmonomers and an improved whiteness.

The present invention further provides water-absorbent polymer particleshaving a bulk density of at least 0.65 g/cm³, a centrifuge retentioncapacity (CRC) at least 35 g/g and a HC value of at least 80.

The present invention further provides fluid-absorbent articles whichcomprise the inventive water-absorbent polymer particles.

DETAILED DESCRIPTION OF THE INVENTION

Preparation of the Water-Absorbent Polymer Particles

The water-absorbent polymer particles are prepared by a process,comprising the steps forming water-absorbent polymer particles bypolymerizing a monomer solution, comprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) at least one crosslinker,-   c₁) less than 0.3% by weight, based on monomer a), of at least one    persulfate,-   c₂) at least 0.05% by weight, based on monomer a), of at least one    azo initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers mentioned under a),-   e) optionally one or more water-soluble polymers, and-   f) water,

in a surrounding heated gas phase in a reaction zone and thermalposttreatment in a fluidized bed, wherein the temperature of the gasleaving the reaction zone is less than 150° C., the temperature of thewater-absorbent polymer particles during the thermal posttreatment isless than 100° C. and the residence time of the water-absorbent polymerparticles in the fluidized bed is from 60 to 300 minutes.

The water-absorbent polymer particles are typically insoluble butswellable in water.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,and itaconic acid. Particularly preferred monomers are acrylic acid andmethacrylic acid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids such as vinylsulfonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities may have a strong impact on the polymerization. Preference isgiven to especially purified monomers a). Useful purification methodsare disclosed in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514A1. A suitable monomer a) is according to WO 2004/035514 A1 purifiedacrylic acid having 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203 by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

Polymerized diacrylic acid is a source for residual monomers due tothermal decomposition. If the temperatures during the process are low,the concentration of diacrylic acid is no more critical and acrylicacids having higher concentrations of diacrylic acid, i.e. 500 to 10,000ppm, can be used for the inventive process.

The content of acrylic acid and/or salts thereof in the total amount ofmonomers a) is preferably at least 50 mol %, more preferably at least 90mol %, most preferably at least 95 mol %.

The acid groups of the monomers a) are typically partly neutralized,preferably to an extent of from 25 to 85 mol %, preferentially to anextent of from 50 to 80 mol %, more preferably from 60 to 75 mol %, forwhich the customary neutralizing agents can be used, preferably alkalimetal hydroxides, alkali metal oxides, alkali metal carbonates or alkalimetal hydrogen carbonates, and mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonia or organic amines, forexample, triethanolamine. It is also possible to use oxides, carbonates,hydrogen-carbonates and hydroxides of magnesium, calcium, strontium,zinc or aluminum as powders, slurries or solutions and mixtures of anyof the above neutralization agents. Example for a mixture is a solutionof sodiumaluminate. Sodium and potassium are particularly preferred asalkali metals, but very particular preference is given to sodiumhydroxide, sodium carbonate or sodium hydrogen carbonate, and mixturesthereof. Typically, the neutralization is achieved by mixing in theneutralizing agent as an aqueous solution, as a melt or preferably alsoas a solid. For example, sodium hydroxide with water contentsignificantly below 50% by weight may be present as a waxy materialhaving a melting point above 23° C. In this case, metered addition aspiece material or melt at elevated temperature is possible.

Optionally, it is possible to add to the monomer solution, or tostarting materials thereof, one or more chelating agents for maskingmetal ions, for example iron, for the purpose of stabilization. Suitablechelating agents are, for example, alkali metal citrates, citric acid,alkali metal tartrates, alkali metal lactates and glycolates,pentasodium triphosphate, ethylenediamine tetraacetate, nitrilotriaceticacid, and all chelating agents known under the Trilon® name, for exampleTrilon® C (pentasodium diethylenetriaminepentaacetate), Trilon® D(trisodium (hydroxyethyl)-ethylenediaminetriacetate), and Trilon® M(methylglycinediacetic acid).

The monomers a) comprise typically polymerization inhibitors, preferablyhydroquinone monoethers, as inhibitor for storage.

The monomer solution comprises preferably up to 250 ppm by weight, morepreferably not more than 130 ppm by weight, most preferably not morethan 70 ppm by weight, preferably not less than 10 ppm by weight, morepreferably not less than 30 ppm by weight and especially about 50 ppm byweight of hydroquinone monoether, based in each case on acrylic acid,with acrylic acid salts being counted as acrylic acid. For example, themonomer solution can be prepared using acrylic acid having appropriatehydroquinone monoether content. The hydroquinone monoethers may,however, also be removed from the monomer solution by absorption, forexample on activated carbon.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized by a free-radical mechanisminto the polymer chain and functional groups which can form covalentbonds with the acid groups of monomer a). In addition, polyvalent metalions which can form coordinate bond with at least two acid groups ofmonomer a) are also suitable crosslinkers b). The crosslinkers b) arepreferably compounds having at least two free-radically polymerizablegroups which can be polymerized by a free-radical mechanism into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and in DE 103 31450 A1, mixed acrylates which, as well as acrylate groups, comprisefurther ethylenically unsaturated groups, as described in DE 103 314 56A1 and DE 103 55 401 A1, or crosslinker mixtures, as described, forexample, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO2002/32962 A2.

Suitable crosslinkers b) are in particular pentaerythritol triallylether, tetraallyloxyethane, polyethyleneglycole diallylethers (based onpolyethylene glycole having a molecular weight between 400 and 20000g/mol), N,N′-methylenebisacrylamide, 15-tuply ethoxylatedtrimethylolpropane, polyethylene glycol diacrylate, trimethylolpropanetriacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 18-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol and especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.05 to 1.5% by weight,more preferably from 0.1 to 1% by weight, most preferably from 0.3 to0.6% by weight, based in each case on monomer a). On increasing theamount of crosslinker b) the centrifuge retention capacity (CRC)decreases and the absorption under a pressure of 21.0 g/cm² (AUL) passesthrough a maximum.

The initiators c) used may be all compounds which disintegrate into freeradicals under the polymerization conditions, for example peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds and redoxinitiators. Preference is given to the use of water-soluble initiators.In some cases, it is advantageous to use mixtures of various initiators,for example mixtures of hydrogen peroxide and sodium or potassiumperoxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate can be used in any proportion. The initiators c) shouldbe water-soluble.

Particularly preferred initiators c) are azo initiators such as2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride,2,2′-azobis(2-amidinopropane)dihydrochloride,4,4′-azobis(4-cyanopentanoic acid), 4,4′-azobis(4-cyanopentanoic acid)sodium salt, 2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide and2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, andphotoinitiators such as 2-hydroxy-2-methylpropiophenone and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redoxinitiators such as sodium persulfate/hydroxymethylsulfinic acid,ammonium peroxodisulfate/hydroxymethylsulfinic acid, hydrogenperoxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid,ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbicacid, photoinitiators such as1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, andmixtures thereof. The reducing component used is, however, preferably amixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, thedisodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite.Such mixtures are obtainable as Brüggolite® FF6 and Brüggolite® FF7(Brüggolite Chemicals; Heilbronn; Germany). Of course it is alsopossible within the scope of the present invention to use the purifiedsalts or acids of 2-hydroxy-2-sulfinatoacetic acid and2-hydroxy-2-sulfonatoacetic acid—the latter being available as sodiumsalt under the trade name Blancolen® (Brüggolite Chemicals; Heilbronn;Germany).

The amount of persulfate c₁) to be used is less than 0.3% by weight,preferably from 0.01 to 0.25% by weight, more preferably from 0.05 to0.2% by weight, most preferably from 0.1 to 0.15% by weight, each basedon monomer a). If the amount of persulfate is too low, a sufficient lowlevel of residual monomers cannot be achieved. If the amount ofpersulfate is too high, the water-absorbent polymer particles do nothave a sufficient whiteness.

The amount of azo initiator c₂) to be used is at least 0.05% by weight,preferably from 0.1 to 2% by weight, more preferably from 0.15 to 1% byweight, most preferably from 0.2 to 0.5% by weight, each based onmonomer a). If the amount of azo initiator is too low, a high centrifugeretention capacity (CRC) cannot be achieved. If the amount of azoinitiator is too high, the process becomes too expensive.

Examples of ethylenically unsaturated monomers d) which arecopolymerizable with the monomers a) are acrylamide, methacrylamide,hydroxyethyl acrylate, hydroxyethyl methacrylate, butanediolmonoacrylate, butanediol monomethacrylate dimethylaminoethyl acrylate,dimethylaminoethyl methacrylate, dimethylaminopropyl acrylate anddiethylaminopropyl methacrylate.

Useful water-soluble polymers e) include polyvinyl alcohol, modifiedpolyvinyl alcohol comprising acidic side groups for example Poval® K(Kuraray Europe GmbH; Frankfurt; Germany), polyvinylpyrrolidone, starch,starch derivatives, modified cellulose such as methylcellulose,carboxymethylcellulose or hydroxyethylcellulose, gelatin, polyglycols orpolyacrylic acids, polyesters and polyamides, polylactic acid,polyvinylamine, polyallylamine, water soluble copolymers of acrylic acidand maleic acid available as Sokalan® (BASF SE; Ludwigshafen; Germany),preferably starch, starch derivatives and modified cellulose.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Therefore, the monomer solution can be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingthrough with an inert gas, preferably nitrogen. It is also possible toreduce the concentration of dissolved oxygen by adding a reducing agent.The oxygen content of the monomer solution is preferably lowered beforethe polymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight.

The water content of the monomer solution is preferably less than 65% byweight, preferentially less than 62% by weight, more preferably lessthan 60% by weight, most preferably less than 58% by weight.

The monomer solution has, at 20° C., a dynamic viscosity of preferablyfrom 0.002 to 0.02 Pa·s, more preferably from 0.004 to 0.015 Pa·s, mostpreferably from 0.005 to 0.01 Pa·s. The mean droplet diameter in thedroplet generation rises with rising dynamic viscosity.

The monomer solution has, at 20° C., a density of preferably from 1 to1.3 g/cm³, more preferably from 1.05 to 1.25 g/cm³, most preferably from1.1 to 1.2 g/cm³.

The monomer solution has, at 20° C., a surface tension of from 0.02 to0.06 N/m, more preferably from 0.03 to 0.05 N/m, most preferably from0.035 to 0.045 N/m. The mean droplet diameter in the droplet generationrises with rising surface tension.

Polymerization

The water-absorbent polymer particles are produced by polymerizingdroplets of the monomer in a surrounding heated gas phase, for exampleusing a system described in WO 2008/040715 A2, WO 2008/052971 A1, WO2008/069639 A1 and WO 2008/086976 A1.

The droplets are preferably generated by means of a droplet plate. Adroplet plate is a plate having a multitude of bores, the liquidentering the bores from the top. The droplet plate or the liquid can beoscillated, which generates a chain of ideally monodisperse droplets ateach bore on the underside of the droplet plate. In a preferredembodiment, the droplet plate is not agitated.

Within the scope of the present invention it is also possible to use twoor more droplet plates with different bore diameters so that a range ofdesired particle sizes can be produced. It is preferable that eachdroplet plate carries only one bore diameter, however mixed borediameters in one plate are also possible.

The number and size of the bores are selected according to the desiredcapacity and droplet size. The droplet diameter is typically 1.9 timesthe diameter of the bore. What is important here is that the liquid tobe dropletized does not pass through the bore too rapidly and thepressure drop over the bore is not too great. Otherwise, the liquid isnot dropletized, but rather the liquid jet is broken up (sprayed) owingto the high kinetic energy. In a preferred embodiment of the presentinvention the pressure drop is from 4 to 5 bar. The Reynolds numberbased on the throughput per bore and the bore diameter is preferablyless than 2000, preferentially less than 1600, more preferably less than1400 and most preferably less than 1200.

The underside of the droplet plate has at least in part a contact anglepreferably of at least 60°, more preferably at least 75° and mostpreferably at least 90° with regard to water.

The contact angle is a measure of the wetting behavior of a liquid, inparticular water, with regard to a surface, and can be determined usingconventional methods, for example in accordance with ASTM D 5725. A lowcontact angle denotes good wetting, and a high contact angle denotespoor wetting.

It is also possible for the droplet plate to consist of a materialhaving a lower contact angle with regard to water, for example a steelhaving the German construction material code number of 1.4571, and becoated with a material having a larger contact angle with regard towater.

Useful coatings include for example fluorous polymers, such asperfluoroalkoxyethylene, polytetrafluoroethylene,ethylene-chlorotrifluoroethylene copolymers,ethylene-tetrafluoroethylene copolymers and fluorinated polyethylene.

The coatings can be applied to the substrate as a dispersion, in whichcase the solvent is subsequently evaporated off and the coating is heattreated. For polytetrafluoroethylene this is described for example inU.S. Pat. No. 3,243,321.

Further coating processes are to be found under the headword “ThinFilms” in the electronic version of “Ullmann's Encyclopedia ofIndustrial Chemistry” (Updated Sixth Edition, 2000 Electronic Release).

The coatings can further be incorporated in a nickel layer in the courseof a chemical nickelization.

It is the poor wettability of the droplet plate that leads to theproduction of monodisperse droplets of narrow droplet size distribution.

The droplet plate has preferably at least 5, more preferably at least25, most preferably at least 50 and preferably up to 750, morepreferably up to 500 bores, most preferably up to 250. The diameter ofthe bores is adjusted to the desired droplet size.

The separation of the bores is usually from 5 to 50 mm, preferably from6 to 40 mm, more preferably from 7 to 30 mm, most preferably from 8 to25 mm. Smaller separations of the bores may cause agglomeration of thepolymerizing droplets.

The diameter of the bores is preferably from 50 to 500 μm, morepreferably from 100 to 300 μm, most preferably from 150 to 250 μm.

For optimizing the average particle diameter, droplet plates withdifferent bore diameters can be used. The variation can be done bydifferent bores on one plate or by using different plates, where theeach plate has a different bore diameter. The average particle sizedistribution can be monomodal, bimodal or multimodal. Most preferably itis monomodal or bimodal.

The temperature of the monomer solution as it passes through the bore ispreferably from 5 to 80° C., more preferably from 10 to 70° C., mostpreferably from 30 to 60° C.

A carrier gas flows through the reaction zone. The carrier gas may beconducted through the reaction zone in cocurrent to the free-fallingdroplets of the monomer solution, i.e. from the top downward. After onepass, the gas is preferably recycled at least partly, preferably to anextent of at least 50%, more preferably to an extent of at least 75%,into the reaction zone as cycle gas. Typically, a portion of the carriergas is discharged after each pass, preferably up to 10%, more preferablyup to 3% and most preferably up to 1%.

The oxygen content of the carrier gas is preferably from 0.1 to 15% byvolume, more preferably from 1 to 10% by volume, most preferably from 2to 7% by weight. In the scope of the present invention it is alsopossible to use a carrier gas which is free of oxygen.

As well as oxygen, the carrier gas preferably comprises nitrogen. Thenitrogen content of the gas is preferably at least 80% by volume, morepreferably at least 90% by volume, most preferably at least 95% byvolume. Other possible carrier gases may be selected from carbondioxide, argon, xenon, krypton, neon, helium, sulfurhexafluoride. Anymixture of carrier gases may be used. The carrier gas may also becomeloaded with water and/or acrylic acid vapors.

The gas velocity is preferably adjusted such that the flow in thereaction zone is directed, for example no convection currents opposed tothe general flow direction are present, and is preferably from 0.1 to2.5 m/s, more preferably from 0.3 to 1.5 m/s, even more preferably from0.5 to 1.2 m/s, most preferably from 0.7 to 0.9 m/s.

The gas entrance temperature, i.e. the temperature with which the gasenters the reaction zone, is preferably from 160 to 200° C., morepreferably from 165 to 195° C., even more preferably from 170 to 190°C., most preferably from 175 to 185° C.

The steam content of the gas that enters the reaction zone is preferablyfrom 0.01 to 0.15 kg per kg dry gas, more from 0.02 to 0.12 kg per kgdry gas, most from 0.03 to 0.10 kg per kg dry gas.

The gas entrance temperature is controlled in such a way that the gasexit temperature, i.e. the temperature with which the gas leaves thereaction zone, is less than 150° C., preferably from 90 to 140° C., morepreferably from 100 to 130° C., even more preferably from 105 to 125°C., most preferably from 110 to 120° C.

The steam content of the gas that leaves the reaction zone is preferablyfrom 0.02 to 0.30 kg per kg dry gas, more from 0.04 to 0.28 kg per kgdry gas, most from 0.05 to 0.25 kg per kg dry gas.

The water-absorbent polymer particles can be divided into threecategories: water-absorbent polymer particles of Type 1 are particleswith one cavity, water-absorbent polymer particles of Type 2 areparticles with more than one cavity, and water-absorbent polymerparticles of Type 3 are solid particles with no visible cavity.

The morphology of the water-absorbent polymer particles can becontrolled by the reaction conditions during polymerization.Water-absorbent polymer particles having a high amount of particles withone cavity (Type 1) can be prepared by using low gas velocities and highgas exit temperatures. Water-absorbent polymer particles having a highamount of particles with more than one cavity (Type 2) can be preparedby using high gas velocities and low gas exit temperatures.

Water-absorbent polymer particles having more than one cavity (Type 2)show an improved mechanical stability.

The reaction can be carried out under elevated pressure or under reducedpressure, preferably from 1 to 100 mbar below ambient pressure, morepreferably from 1.5 to 50 mbar below ambient pressure, most preferablyfrom 2 to 10 mbar below ambient pressure.

The reaction off-gas, i.e. the gas leaving the reaction zone, may becooled in a heat exchanger. This condenses water and unconverted monomera). The reaction off-gas can then be reheated at least partly andrecycled into the reaction zone as cycle gas. A portion of the reactionoff-gas can be discharged and replaced by fresh gas, in which case waterand unconverted monomers a) present in the reaction off-gas can beremoved and recycled.

Particular preference is given to a thermally integrated system, i.e. aportion of the waste heat in the cooling of the off-gas is used to heatthe cycle gas.

Deposits on the reactor walls can be removed by use of any availablevibrating or knocking means.

The reactors can be trace-heated. In this case, the trace heating isadjusted such that the wall temperature is at least 5° C. above theinternal surface temperature and condensation on the surfaces isreliably prevented.

Thermal Posttreatment

The formed water-absorbent polymer particles are thermal posttreated ina fluidized bed. In a preferred embodiment of the present invention aninternal fluidized bed is used. An internal fluidized bed means that theproduct of the dropletization polymerization is accumulated in afluidized bed below the reaction zone.

The residual monomers can be removed during the thermal posttreatment.What is important here is that the water-absorbent polymer particles arenot too dry. In the case of excessively dry particles, the residualmonomers decrease only insignificantly. Too high a water contentincreases the caking tendency of the water-absorbent polymer particles.

In the fluidized state, the kinetic energy of the polymer particles isgreater than the cohesion or adhesion potential between the polymerparticles.

The fluidized state can be achieved by a fluidized bed. In this bed,there is upward flow toward the water-absorbing polymer particles, sothat the particles form a fluidized bed. The height of the fluidized bedis adjusted by gas rate and gas velocity, i.e. via the pressure drop ofthe fluidized bed (kinetic energy of the gas).

The velocity of the gas stream in the fluidized bed is preferably from0.3 to 2.5 m/s, more preferably from 0.4 to 2.0 m/s, most preferablyfrom 0.5 to 1.5 m/s.

The pressure drop over the bottom of the internal fluidized bed ispreferably from 1 to 100 mbar, more preferably from 3 to 50 mbar, mostpreferably from 5 to 25 mbar.

The moisture content of the water-absorbent polymer particles at the endof the thermal post-treatment is preferably from 1 to 20% by weight,more preferably from 2 to 15% by weight, even more preferably from 3 to12% by weight, most preferably 5 to 8% by weight.

The temperature of the water-absorbent polymer particles during thethermal posttreatment is from 20 to 120° C., preferably from 40 to 100°C., more preferably from 50 to 95° C., even more preferably from 55 to90° C., most preferably from 60 to 80° C.

The average residence time in the internal fluidized bed is from 10 to300 minutes, preferably from 60 to 270 minutes, more preferably from 40to 250 minutes, most preferably from 120 to 240 minutes.

The condition of the fluidized bed can be adjusted that the level ofresidual monomers of the water-absorbent polymers leaving the fluidizedbed is preferably from 0.1 to 5% by weight, more preferably from 0.15 to3% by weight, most preferably from 0.2 to 2% by weight.

The steam content of the gas is preferably from 0.005 to 0.25 kg per kgof dry gas, more preferably from 0.01 to 0.2 kg per kg of dry gas, mostpreferably from 0.02 to 0.15 kg per kg of dry gas.

By using additional steam the condition of the fluidized bed can beadjusted that the level of residual monomers of the water-absorbentpolymers leaving he internal fluidized bed is preferably from 0.0001 to1% by weight, more preferably from 0.0005 to 0.5% by weight, mostpreferably from 0.001 to 0.2% by weight.

In one preferred embodiment of the present invention the thermalposttreatment is completely or at least partially done in an externalfluidized bed. The operating conditions of the external fluidized bedare within the scope for the internal fluidized bed as described above.

The level of residual monomers can be further reduced by an additionalthermal posttreatment in a mixer with rotating mixing tools as describedin WO 211/117215 A1.

The morphology of the water-absorbent polymer particles can also becontrolled by the reaction conditions during thermal posttreatment.Water-absorbent polymer particles having a high amount of particles withone cavity (Type 1) can be prepared by using high product temperaturesand short residence times. Water-absorbent polymer particles having ahigh amount of particles with more than one cavity (Type 2) can beprepared by using low product temperatures and long residence times.

The present invention is based upon the fact that the thermalposttreatment has a strong impact on the morphology of the formedwater-absorbent polymer particles. Water-absorbent polymer particleshaving superior properties can be produced by adjusting the conditionsof the thermal posttreatment.

Surface-Postcrosslinking

In the present invention the water-absorbent polymer particles may besurface-postcrosslinked for further improvement of the properties.

Surface-postcrosslinkers are compounds which comprise groups which canform at least two covalent bonds with the carboxylate groups of thepolymer particles. Suitable compounds are, for example, polyfunctionalamines, polyfunctional amidoamines, polyfunctional epoxides, asdescribed in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di-or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 10204 938 A1 and U.S. Pat. No. 6,239,230. Also ethyleneoxide, aziridine,glycidol, oxetane and its derivatives may be used.

Polyvinylamine, polyamidoamines and polyvinylalcohole are examples ofmultifunctional polymeric surface-postcrosslinkers.

In addition, DE 40 20 780 C1 describes cyclic carbonates, DE 198 07 502A1 describes 2-oxazolidone and its derivatives such as2-hydroxyethyl-2-oxazolidone, DE 198 07 992 C1 describes bis- andpoly-2-oxazolidinones, DE 198 54 573 A1 describes2-oxotetrahydro-1,3-oxazine and its derivatives, DE 198 54 574 A1describes N-acyl-2-oxazolidones, DE 102 04 937 A1 describes cyclicureas, DE 103 34 584 A1 describes bicyclic amide acetals, EP 1 199 327A2 describes oxetanes and cyclic ureas, and WO 2003/31482 A1 describesmorpholine-2,3-dione and its derivatives, as suitablesurface-postcrosslinkers.

Particularly preferred postcrosslinkers are ethylene carbonate,glycerine carbonate, mixtures of propylene glycol, 1,3-propandiole,1,4-butanediol, mixtures of 1,3-propandiole and 1,4-butanediole,ethylene glycol diglycidyl ether and reaction products of polyamides andepichlorohydrin.

Very particularly preferred postcrosslinkers are2-hydroxyethyl-2-oxazolidone, 2-oxazolidone, ethylene carbonate and1,3-propanediol.

In addition, it is also possible to use surface-postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

The amount of postcrosslinker is preferably from 0.001 to 2% by weight,more preferably from 0.02 to 1% by weight, most preferably from 0.05 to0.2% by weight, based in each case on the polymer.

The moisture content of the water-absorbent polymer particles prior tothe thermal surface-postcrosslinking is preferably from 1 to 20% byweight, more preferably from 2 to 15% by weight, most preferably from 3to 10% by weight.

The amount of alkylene carbonate is preferably from 0.1 to 10% byweight, more preferably from 0.5 to 7.5% by weight, most preferably from1.0 to 5% by weight, based in each case on the polymer.

The content of residual monomers in the water-absorbent polymerparticles prior to the coating with the alkylene carbonate is in therange from 0.1 to 10% by weight, preferably from 0.15 to 7.5% by weight,more preferably from 0.2 to 5% by weight, most preferably from 0.25 to2.5% by weight.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to thesurface-postcrosslinkers before, during or after the thermalsurface-postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium, andmixtures thereof. Possible counterions are chloride, bromide, sulfate,hydrogensulfate, carbonate, hydrogencarbonate, nitrate, hydroxide,phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, suchas acetate, glycolate, tartrate, formiate, propionate,3-hydroxypropionate, lactamide and lactate, and mixtures thereof.Aluminum sulfate, aluminum acetate, and aluminum lactate are preferred.Apart from metal salts, it is also possible to use polyamines and/orpolymeric amines as polyvalent cations. A single metal salt can be usedas well as any mixture of the metal salts and/or the polyamines above.

Preferred polyvalent cations and corresponding anions are disclosed inWO 2012/045705 A1 and are expressly incorporated herein by reference.Preferred polyvinylamines are disclosed in WO 2004/024816 A1 and areexpressly incorporated herein by reference.

The amount of polyvalent cation used is, for example, from 0.001 to 1.5%by weight, preferably from 0.005 to 1% by weight, more preferably from0.02 to 0.8% by weight, based in each case on the polymer.

The addition of the polyvalent metal cation can take place prior, after,or cocurrently with the surface-postcrosslinking. Depending on theformulation and operating conditions employed it is possible to obtain ahomogeneous surface coating and distribution of the polyvalent cation oran inhomogenous typically spotty coating. Both types of coatings and anymixes between them are useful within the scope of the present invention.

The surface-postcrosslinking is typically performed in such a way that asolution of the surface-postcrosslinker is sprayed onto the hydrogel orthe dry polymer particles. After the spraying, the polymer particlescoated with the surface-postcrosslinker are dried thermally and cooled.

The spraying of a solution of the surface-postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers and paddle mixers. Suitable mixers are, for example, verticalSchugi Flexomix® mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), Turbolizers® mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), horizontal Pflugschar® plowshare mixers (Gebr. LödigeMaschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixers(Hosokawa Micron BV; Doetinchem; the Netherlands), Processall MixmillMixers (Processall Incorporated; Cincinnati; US) and Ruberg continuousflow mixers (Gebrider Ruberg GmbH & Co KG, Nieheim, Germany). Rubergcontinuous flow mixers and horizontal Pflugschar® plowshare mixers arepreferred. The surface-postcrosslinker solution can also be sprayed intoa fluidized bed.

The solution of the surface-postcrosslinker can also be sprayed on thewater-absorbent polymer particles during the thermal posttreatment. Insuch case the surface-postcrosslinker can be added as one portion or inseveral portions along the axis of thermal posttreatment mixer. In oneembodiment it is preferred to add the surface-postcrosslinker at the endof the thermal post-treatment step. As a particular advantage of addingthe solution of the surface-postcrosslinker during the thermalposttreatment step it may be possible to eliminate or reduce thetechnical effort for a separate surface-postcrosslinker addition mixer.

The surface-postcrosslinkers are typically used as an aqueous solution.The addition of nonaqueous solvent can be used to adjust the penetrationdepth of the surface-postcrosslinker into the polymer particles.

The thermal surface-postcrosslinking is preferably carried out incontact dryers, more preferably paddle dryers, most preferably diskdryers. Suitable driers are, for example, Hosokawa Bepex® horizontalpaddle driers (Hosokawa Micron GmbH; Leingarten; Germany), HosokawaBepex® disk driers (Hosokawa Micron GmbH; Leingarten; Germany),Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; U.S.A.)and Nara paddle driers (NARA Machinery Europe; Frechen; Germany). Narapaddle driers and, in the case of low process temperatures (<160° C.)for example, when using polyfunctional epoxides, Holo-Flite® dryers arepreferred. Moreover, it is also possible to use fluidized bed dryers. Inthe latter case the reaction times may be shorter compared to otherembodiments.

When a horizontal dryer is used then it is often advantageous to set thedryer up with an inclined angle of a few degrees vs. the earth surfacein order to impart proper product flow through the dryer. The angle canbe fixed or may be adjustable and is typically between 0 to 10 degrees,preferably 1 to 6 degrees, most preferably 2 to 4 degrees.

In one embodiment of the present invention a contact dryer is used thathas two different heating zones in one apparatus. For example Narapaddle driers are available with just one heated zone or alternativelywith two heated zones. The advantage of using a two or more heated zonedryer is that different phases of the thermal post-treatment and/or ofthe post-surface-crosslinking can be combined.

In one preferred embodiment of the present invention a contact dryerwith a hot first heating zone is used which is followed by a temperatureholding zone in the same dryer. This set up allows a quick rise of theproduct temperature and evaporation of surplus liquid in the firstheating zone, whereas the rest of the dryer is just holding the producttemperature stable to complete the reaction.

In another preferred embodiment of the present invention a contact dryerwith a warm first heating zone is used which is then followed by a hotheating zone. In the first warm zone the thermal post-treatment isaffected or completed whereas the surface-postcrosslinking takes placein the subsequential hot zone.

In a typical embodiment a paddle heater with just one temperature zoneis employed.

A person skilled in the art will depending on the desired finishedproduct properties and the available base poylmer qualities from thepolymerization step choose any one of these set ups.

The thermal surface-postcrosslinking can be effected in the mixeritself, by heating the jacket, blowing in warm air or steam. Equallysuitable is a downstream dryer, for example a shelf dryer, a rotary tubeoven or a heatable screw. It is particularly advantageous to mix and dryin a fluidized bed dryer.

Preferred thermal surface-postcrosslinking temperatures are in the rangefrom 100 to 180° C., preferably from 120 to 170° C., more preferablyfrom 130 to 165° C., most preferably from 140 to 160° C. The preferredresidence time at this temperature in the reaction mixer or dryer ispreferably at least 5 minutes, more preferably at least 20 minutes, mostpreferably at least 40 minutes, and typically at most 120 minutes.

It is preferable to cool the polymer particles after thermalsurface-postcrosslinking. The cooling is preferably carried out incontact coolers, more preferably paddle coolers, most preferably diskcoolers. Suitable coolers are, for example, Hosokawa Bepex® horizontalpaddle coolers (Hosokawa Micron GmbH; Leingarten; Germany), HosokawaBepex® disk coolers (Hosokawa Micron GmbH; Leingarten; Germany),Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; U.S.A.)and Nara paddle coolers (NARA Machinery Europe; Frechen; Germany).Moreover, it is also possible to use fluidized bed coolers.

In the cooler the polymer particles are cooled to temperatures in therange from 20 to 150° C., preferably from 40 to 120° C., more preferablyfrom 60 to 100° C., most preferably from 70 to 90° C. Cooling using warmwater is preferred, especially when contact coolers are used.

Coating

To improve the properties, the water-absorbent polymer particles can becoated and/or optionally moistened. The internal fluidized bed, theexternal fluidized bed and/or the external mixer used for the thermalposttreatment and/or a separate coater (mixer) can be used for coatingof the water-absorbent polymer particles. Further, the cooler and/or aseparate coater (mixer) can be used for coating/moistening of thesurface-postcrosslinked water-absorbent polymer particles. Suitablecoatings for controlling the acquisition behavior and improving thepermeability (SFC or GBP) are, for example, inorganic inert substances,such as water-insoluble metal salts, organic polymers, cationicpolymers, anionic polymers and polyvalent metal cations. Suitablecoatings for improving the color stability are, for example reducingagents, chelating agents and anti-oxidants. Suitable coatings for dustbinding are, for example, polyols. Suitable coatings against theundesired caking tendency of the polymer particles are, for example,fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.Preferred coatings are aluminium dihydroxy monoacetate, aluminiumsulfate, aluminium lactate, aluminium 3-hydroxypropionate, zirconiumacetate, citric acid or its water soluble salts, di- and mono-phosphoricacid or their water soluble salts, Blancolen®, Brüggolite® FF7, Cublen®and Span® 20.

If salts of the above acids are used instead of the free acids then thepreferred salts are alkali-metal, earth alkali metal, aluminum,zirconium, titanium, zinc and ammonium salts.

Under the trade name Cublen® (Zschimmer & Schwarz Mohsdorf GmbH & Co KG;Burgstadt; Germany) the following acids and/or their alkali metal salts(preferably Na and K-salts) are available and may be used within thescope of the present invention for example to impart color-stability tothe finished product:

1-Hydroxyethane-1,1-diphosphonic acid, Amino-tris(methylene phosphonicacid), Ethylenediamine-tetra(methylene phosphonic acid),Diethylenetriamine-penta(methylene phosphonic acid), Hexamethylenediamine-tetra(methylenephosphonic acid), Hydroxyethyl-amino-di(methylenephosphonic acid), 2-Phosphonobutane-1,2,4-tricarboxylic acid,Bis(hexamethylenetriamine penta(methylene phosphonic acid)).

Most preferably 1-Hydroxyethane-1,1-diphosphonic acid or its salts withsodium, potassium, or ammonium are employed. Any mixture of the aboveCublenes® can be used.

Alternatively, any of the chelating agents described before for use inthe polymerization can be coated onto the finished product.

Suitable inorganic inert substances are silicates such asmontmorillonite, kaolinite and talc, zeolites, activated carbons,polysilicic acids, magnesium carbonate, calcium carbonate, calciumphosphate, barium sulfate, aluminum oxide, titanium dioxide and iron(II)oxide. Preference is given to using polysilicic acids, which are dividedbetween precipitated silicas and fumed silicas according to their modeof preparation. The two variants are commercially available under thenames Silica FK, Sipernat®, Wessalon® (precipitated silicas) andAerosil® (fumed silicas) respectively. The inorganic inert substancesmay be used as dispersion in an aqueous or water-miscible dispersant orin substance.

When the water-absorbent polymer particles are coated with inorganicinert substances, the amount of inorganic inert substances used, basedon the water-absorbent polymer particles, is preferably from 0.05 to 5%by weight, more preferably from 0.1 to 1.5% by weight, most preferablyfrom 0.3 to 1% by weight.

Suitable organic polymers are polyalkyl methacrylates or thermoplasticssuch as polyvinyl chloride, waxes based on polyethylene, polypropylene,polyamides or polytetrafluoro-ethylene. Other examples arestyrene-isoprene-styrene block-copolymers or styrene-butadiene-styreneblock-copolymers. Another example are silanole-group bearingpolyvinylalcoholes available under the trade name Poval® R (KurarayEurope GmbH; Frankfurt; Germany).

Suitable cationic polymers are polyalkylenepolyamines, cationicderivatives of polyacrylamides, polyethyleneimines and polyquaternaryamines.

Polyquaternary amines are, for example, condensation products ofhexamethylenediamine, dimethylamine and epichlorohydrin, condensationproducts of dimethylamine and epichlorohydrin, copolymers ofhydroxyethylcellulose and diallyldimethylammonium chloride, copolymersof acrylamide and α-methacryloyloxyethyltrimethylammonium chloride,condensation products of hydroxyethylcellulose, epichlorohydrin andtrimethylamine, homopolymers of diallyldimethylammonium chloride andaddition products of epichlorohydrin to amidoamines. In addition,polyquaternary amines can be obtained by reacting dimethyl sulfate withpolymers such as polyethyleneimines, copolymers of vinylpyrrolidone anddimethylaminoethyl methacrylate or copolymers of ethyl methacrylate anddiethylaminoethyl methacrylate. The polyquaternary amines are availablewithin a wide molecular weight range.

However, it is also possible to generate the cationic polymers on theparticle surface, either through reagents which can form a network withthemselves, such as addition products of epichlorohydrin topolyamidoamines, or through the application of cationic polymers whichcan react with an added crosslinker, such as polyamines or polyimines incombination with polyepoxides, polyfunctional esters, polyfunctionalacids or polyfunctional (meth)acrylates.

It is possible to use all polyfunctional amines having primary orsecondary amino groups, such as polyethyleneimine, polyallylamine andpolylysine. The liquid sprayed by the process according to the inventionpreferably comprises at least one polyamine, for example polyvinylamineor a partially hydrolyzed polyvinylformamide.

The cationic polymers may be used as a solution in an aqueous orwater-miscible solvent, as dispersion in an aqueous or water-miscibledispersant or in substance.

When the water-absorbent polymer particles are coated with a cationicpolymer, the use amount of cationic polymer based on the water-absorbentpolymer particles is usually not less than 0.001% by weight, typicallynot less than 0.01% by weight, preferably from 0.1 to 15% by weight,more preferably from 0.5 to 10% by weight, most preferably from 1 to 5%by weight.

Suitable anionic polymers are polyacrylates (in acidic form or partiallyneutralized as salt), copolymers of acrylic acid and maleic acidavailable under the trade name Sokalan® (BASF SE; Ludwigshafen;Germany), and polyvinylalcohols with built in ionic charges availableunder the trade name Poval® K (Kuraray Europe GmbH; Frankfurt; Germany).

Suitable polyvalent metal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺,Mn²⁺, Fe^(2+/3+), Co²⁺, Ni²⁺, Cu^(+/2+), Zn²⁺, y³⁺, Zr⁴⁺, Ag⁺, La³⁺,Ce⁴⁺, Hf⁴⁺ and Au^(+/3+); preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺,Ti⁴⁺, Zr⁴⁺ and La³⁺; particularly preferred metal cations are Al³⁺, Ti⁴⁺and Zr⁴⁺. The metal cations may be used either alone or in a mixturewith one another. Suitable metal salts of the metal cations mentionedare all of those which have a sufficient solubility in the solvent to beused. Particularly suitable metal salts have weakly complexing anions,such as chloride, hydroxide, carbonate, acetate, formiate, propionate,nitrate and sulfate. The metal salts are preferably used as a solutionor as a stable aqueous colloidal dispersion. The solvents used for themetal salts may be water, alcohols, ethylene carbonate, propylenecarbonate, dimethylformamide, dimethyl sulfoxide and mixtures thereof.Particular preference is given to water and water/alcohol mixtures, suchas water/methanol, water/isopropanol, water/1,3-propanediole,water/1,2-propandiole/1,4-butanediole or water/propylene glycol.

When the water-absorbent polymer particles are coated with a polyvalentmetal cation, the amount of polyvalent metal cation used, based on thewater-absorbent polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Suitable reducing agents are, for example, sodium sulfite, sodiumhydrogensulfite (sodium bisulfite), sodium dithionite, sulfinic acidsand salts thereof, ascorbic acid, sodium hypophosphite, sodiumphosphite, and phosphinic acids and salts thereof. Preference is given,however, to salts of hypophosphorous acid, for example sodiumhypophosphite, salts of sulfinic acids, for example the disodium salt of2-hydroxy-2-sulfinatoacetic acid, and addition products of aldehydes,for example the disodium salt of 2-hydroxy-2-sulfonatoacetic acid. Thereducing agent used can be, however, a mixture of the sodium salt of2-hydroxy-2-sulfinatoacetic acid, the disodium salt of2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures areobtainable as Brüggolite® FF6 and Brüggolite® FF7 (Briggemann Chemicals;Heilbronn; Germany). Also useful is the purified2-hydroxy-2-sulfonatoacetic acid and its sodium salts, available underthe trade name Blancolen® from the same company.

The reducing agents are typically used in the form of a solution in asuitable solvent, preferably water. The reducing agent may be used as apure substance or any mixture of the above reducing agents may be used.

When the water-absorbent polymer particles are coated with a reducingagent, the amount of reducing agent used, based on the water-absorbentpolymer particles, is preferably from 0.01 to 5% by weight, morepreferably from 0.05 to 2% by weight, most preferably from 0.1 to 1% byweight.

Suitable polyols are polyethylene glycols having a molecular weight offrom 400 to 20000 g/mol, polyglycerol, 3- to 100-tuply ethoxylatedpolyols, such as trimethylolpropane, glycerol, sorbitol and neopentylglycol. Particularly suitable polyols are 7- to 20-tuply ethoxylatedglycerol or trimethylolpropane, for example Polyol TP 70® (Perstorp AB,Perstorp, Sweden). The latter have the advantage in particular that theylower the surface tension of an aqueous extract of the water-absorbentpolymer particles only insignificantly. The polyols are preferably usedas a solution in aqueous or water-miscible solvents.

When the water-absorbent polymer particles are coated with a polyol, theuse amount of polyol, based on the water-absorbent polymer particles, ispreferably from 0.005 to 2% by weight, more preferably from 0.01 to 1%by weight, most preferably from 0.05 to 0.5% by weight.

The coating is preferably performed in mixers with moving mixing tools,such as screw mixers, disk mixers, paddle mixers and drum coater.Suitable mixers are, for example, horizontal Pflugschar® plowsharemixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany),Vrieco-Nauta Continuous Mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), Processall Mixmill Mixers (Processall Incorporated;Cincinnati; US) and Ruberg continuous flow mixers (Gebrider Ruberg GmbH& Co KG, Nieheim, Germany). Moreover, it is also possible to use afluidized bed for mixing.

Agglomeration

The water-absorbent polymer particles can further selectivily beagglomerated. The agglomeration can take place after the polymerization,the thermal postreatment, the thermal surface-postcrosslinking or thecoating.

Useful agglomeration assistants include water and water-miscible organicsolvents, such as alcohols, tetrahydrofuran and acetone; water-solublepolymers can be used in addition.

For agglomeration a solution comprising the agglomeration assistant issprayed onto the water-absorbing polymeric particles. The spraying withthe solution can, for example, be carried out in mixers having movingmixing implements, such as screw mixers, paddle mixers, disk mixers,plowshare mixers and shovel mixers. Useful mixers include for exampleLödige® mixers, Bepex® mixers, Nauta® mixers, Processall® mixers andSchugi® mixers. Vertical mixers are preferred. Fluidized bed apparatusesare particularly preferred.

Combination of thermal posttreatment, surface-postcrosslinking andoptionally coating

In a preferred embodiment of the present invention the steps of thermalposttreatment and thermal surface-postcrosslinking are combined in oneprocess step. Such combination allows the use of low cost equipment andmoreover the process can be run at low temperatures, that iscost-efficient and avoids discoloration and loss of performanceproperties of the finished product by thermal degradation.

The mixer may be selected from any of the equipment options cited in thethermal posttreatment section. Ruberg continuous flow mixers, Beckershovel mixers and Pflugschar® plowshare mixers are preferred.

In this particular preferred embodiment the surface-postcrosslinkingsolution is sprayed onto the water-absorbent polymer particles underagitation.

Following the thermal posttreatment/surface-postcrosslinking thewater-absorbent polymer particles are dried to the desired moisturelevel and for this step any dryer cited in the surface-postcrosslinkingsection may be selected. However, as only drying needs to beaccomplished in this particular preferred embodiment it is possible touse simple and low cost heated contact dryers like a heated screw dryer,for example a Holo-Flite® dryer (Metso Minerals Industries Inc.;Danville; U.S.A.). Alternatively a fluidized bed may be used. In caseswhere the product needs to be dried with a predetermined and narrowresidence time it is possible to use torus disc dryers or paddle dryers,for example a Nara paddle dryer (NARA Machinery Europe; Frechen;Germany).

In a preferred embodiment of the present invention, polyvalent cationscited in the surface-postcrosslinking section are applied to theparticle surface before, during or after addition of thesurface-postcrosslinker by using different addition points along theaxis of a horizontal mixer.

In a very particular preferred embodiment of the present invention thesteps of thermal post-treatment, surface-postcrosslinking, and coatingare combined in one process step. Suitable coatings are cationicpolymers, surfactants, and inorganic inert substances that are cited inthe coating section. The coating agent can be applied to the particlesurface before, during or after addition of the surface-postcrosslinkeralso by using different addition points along the axis of a horizontalmixer.

The polyvalent cations and/or the cationic polymers can act asadditional scavengers for residual surface-postcrosslinkers. In apreferred embodiment of the present invention thesurface-postcrosslinkers are added prior to the polyvalent cationsand/or the cationic polymers to allow the surface-postcrosslinker toreact first.

The surfactants and/or the inorganic inert substances can be used toavoid sticking or caking during this process step under humidatmospheric conditions. A preferred surfactant is Span® 20. Preferredinorganic inert substances are precipitated silicas and fumed silcas inform of powder or dispersion.

The amount of total liquid used for preparing the solutions/dispersionsis typically from 0.01% to 25% by weight, preferably from 0.5% to 12% byweight, more preferably from 2% to 7% by weight, most preferably from 3%to 6% by weight, in respect to the weight amount of water-absorbentpolymer particles to be processed.

Preferred embodiments are depicted in FIGS. 1 to 12.

FIG. 1: Process scheme (without external fluidized bed)

FIG. 2: Process scheme (with external fluidized bed)

FIG. 3: Arrangement of the T_outlet measurement

FIG. 4: Arrangement of the dropletizer units

FIG. 5: Dropletizer unit (longitudinal cut)

FIG. 6: Dropletizer unit (cross sectional view)

FIG. 7: Bottom of the internal fluidized bed (top view)

FIG. 8: openings in the bottom of the internal fluidized bed

FIG. 9: Rake stirrer for the intern fluidized bed (top view)

FIG. 10: Rake stirrer for the intern fluidized bed (cross sectionalview)

FIG. 11: Process scheme (surface-postcrosslinking)

FIG. 12: Process scheme (surface-postcrosslinking and coating)

The reference numerals have the following meanings:

-   -   1 Drying gas inlet pipe    -   2 Drying gas amount measurement    -   3 Gas distributor    -   4 Dropletizer units    -   5 Cocurrent spray dryer, cylindrical part    -   6 Cone    -   7 T_outlet measurement    -   8 Tower offgas pipe    -   9 Baghouse filter    -   10 Ventilator    -   11 Quench nozzles    -   12 Condenser column, counter current cooling    -   13 Heat exchanger    -   14 Pump    -   15 Pump    -   16 Water outlet    -   17 Ventilator    -   18 Offgas outlet    -   19 Nitrogen inlet    -   20 Heat exchanger    -   21 Ventilator    -   22 Heat exchanger    -   23 Steam injection via nozzles    -   24 Water loading measurement    -   25 Conditioned internal fluidized bed gas    -   26 Internal fluidized bed product temperature measurement    -   27 Internal fluidized bed    -   28 Rotary valve    -   29 Sieve    -   30 End product    -   31 Static mixer    -   32 Static mixer    -   33 Initiator feed    -   34 Initiator feed    -   35 Monomer feed    -   36 Fine particle fraction outlet to rework    -   37 External fluidized bed    -   38 Ventilator    -   39 External fluidized bed offgas outlet to baghouse filter    -   40 Rotary valve    -   41 Filtered air inlet    -   42 Ventilator    -   43 Heat exchanger    -   44 Steam injection via nozzle    -   45 Water loading measurement    -   46 Conditioned external fluidized bed gas    -   47 T_outlet measurement (average temperature out of 3        measurements around tower circumference)    -   48 Dropletizer unit    -   49 Monomer premixed with initiator feed    -   50 Spray dryer tower wall    -   51 Dropletizer unit outer pipe    -   52 Dropletizer unit inner pipe    -   53 Dropletizer cassette    -   54 Teflon block    -   55 Valve    -   56 Monomer premixed with initiator feed inlet pipe connector    -   57 Droplet plate    -   58 Counter plate    -   59 Flow channels for temperature control water    -   60 Dead volume free flow channel for monomer solution    -   61 Dropletizer cassette stainless steel block    -   62 Bottom of the internal fluidized bed with four segments    -   63 Split openings of the segments    -   64 Rake stirrer    -   65 Prongs of the rake stirrer    -   66 Mixer    -   67 Optional coating feed    -   68 Postcrosslinker feed    -   69 Thermal dryer (surface-postcrosslinking)    -   70 Cooler    -   71 Optional coating/water feed    -   72 Coater    -   73 Coating/water feed

The drying gas is fed via a gas distributor (3) at the top of the spraydryer as shown in FIG. 1. The drying gas is partly recycled (drying gasloop) via a baghouse filter (9) and a condenser column (12). Thepressure inside the spray dryer is below ambient pressure.

The spray dryer outlet temperature is preferably measured at threepoints around the circumference at the end of the cylindrical part asshown in FIG. 3. The single measurements (47) are used to calculate theaverage cylindrical spray dryer outlet temperature.

The product accumulated in the internal fluidized bed (27). Conditionedinternal fluidized bed gas is fed to the internal fluidized bed (27) vialine (25). The relative humidity of the internal fluidized bed gas ispreferably controlled by adding steam via line (23).

The spray dryer offgas is filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. After the baghouse filter(9) a recuperation heat exchanger system for preheating the gas afterthe condenser column (12) can be used. The baghouse filter (9) may betrace-heated on a temperature of preferably from 80 to 180° C., morepreferably from 90 to 150° C., most preferably from 100 to 140° C.Excess water is pumped out of the condenser column (12) by controllingthe (constant) filling level inside the condenser column (12). The waterinside the condenser column (12) is cooled by a heat exchanger (13) andpumped counter-current to the gas via quench nozzles (11) so that thetemperature inside the condenser column (12) is preferably from 20 to100° C., more preferably from 25 to 80° C., most preferably from 30 to60° C. The water inside the condenser column (12) is set to an alkalinepH by dosing a neutralizing agent to wash out vapors of monomer a).Aqueous solution from the condenser column (12) can be sent back forpreparation of the monomer solution.

The condenser column offgas is split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures are controlled via heat exchangers (20) and (22). The hotdrying gas is fed to the cocurrent spray dryer via gas distributor (3).The gas distributor (3) consists preferably of a set of plates providinga pressure drop of preferably 1 to 100 mbar, more preferably 2 to 30mbar, most preferably 4 to 20 mbar, depending on the drying gas amount.Turbulences and/or a centrifugal velocity can also be introduced intothe drying gas if desired by using gas nozzles or baffle plates.

Conditioned internal fluidized bed gas is fed to the internal fluidizedbed (27) via line (25). The relative humidity of the external fluidizedbed gas is preferably controlled by adding steam via line (23). Toprevent any condensation the steam is added together with the internalfluidized bed into the heat exchanger (22). The product holdup in theinternal fluidized bed (27) can be controlled via rotational speed ofthe rotary valve (28).

The product is discharged from the internal fluidized bed (27) viarotary valve (28). The product holdup in the internal fluidized bed (27)can be controlled via rotational speed of the rotary valve (28). Thesieve (29) is used for sieving off overs/lumps.

The monomer solution is preferably prepared by mixing first monomer a)with a neutralization agent and secondly with crosslinker b). Thetemperature during neutralization is controlled to preferably from 5 to60° C., more preferably from 8 to 40° C., most preferably from 10 to 30°C., by using a heat exchanger and pumping in a loop. A filter unit ispreferably used in the loop after the pump. The initiators are meteredinto the monomer solution upstream of the dropletizer by means of staticmixers (31) and (32) via lines (33) and (34) as shown in FIG. 1.Preferably a peroxide solution having a temperature of preferably from 5to 60° C., more preferably from 10 to 50° C., most preferably from 15 to40° C., is added via line (33) and preferably an azo initiator solutionhaving a temperature of preferably from 2 to 30° C., more preferablyfrom 3 to 15° C., most preferably from 4 to 8° C., is added via line(34). Each initiator is preferably pumped in a loop and dosed viacontrol valves to each dropletizer unit. A second filter unit ispreferably used after the static mixer (32). The mean residence time ofthe monomer solution admixed with the full initiator package in thepiping before the droplet plates (57) is preferably less than 60 s, morepreferably less than 30 s, most preferably less than 10 s.

For dosing the monomer solution into the top of the spray dryerpreferably three dropletizer units are used as shown in FIG. 4. However,any number of dropletizers can be used that is required to optimize thethroughput of the process and the quality of the product. Hence, in thepresent invention at least one dropletizer is employed, and as manydropletizers as geometrically allowed may be used.

A dropletizer unit consists of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) is connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) is controlled topreferably 5 to 80° C., more preferably 10 to 70° C., most preferably 30to 60° C., by water in flow channels (59) as shown in FIG. 6.

The dropletizer cassette has preferably from 10 to 1500, more preferablyfrom 50 to 1000, most preferably from 100 to 500, bores having adiameter of preferably from 50 to 500 μm, more preferably from 100 to300 μm, most preferably from 150 to 250 μm. The bores can be ofcircular, rectangular, triangular or any other shape. Circular bores arepreferred. The ratio of bore length to bore diameter is preferably from0.5 to 10, more preferably from 0.8 to 5, most preferably from 1 to 3.The droplet plate (57) can have a greater thickness than the bore lengthwhen using an inlet bore channel. The droplet plate (57) is preferablylong and narrow as disclosed in WO 2008/086976 A1. Multiple rows ofbores per droplet plate can be used, preferably from 1 to 20 rows, morepreferably from 2 to 5 rows.

The dropletizer cassette (61) consists of a flow channel (60) havingessential no stagnant volume for homogeneous distribution of thepremixed monomer and initiator solutions and two droplet plates (57).The droplet plates (57) have an angled configuration with an angle ofpreferably from 1 to 90°, more preferably from 3 to 45°, most preferablyfrom 5 to 20°. Each droplet plate (57) is preferably made of a heatand/or chemically resistant material, such as stainless steel, polyetherether ketone, polycarbonate, polyarylsulfone, such as polysulfone, orpolyphenylsulfone, or fluorous polymers, such asperfluoroalkoxyethylene, polytetrafluoroethylene, polyvinylidenfluorid,ethylene-chlorotrifluoroethylene copolymers,ethylene-tetrafluoroethylene copolymers and fluorinated polyethylene.Coated droplet plates as disclosed in WO 2007/031441 A1 can also beused. The choice of material for the droplet plate is not limited exceptthat droplet formation must work and it is preferable to use materialswhich do not catalyze the start of polymerization on its surface.

The throughput of monomer including initiator solutions per dropletizerunit is preferably from 150 to 2500 kg/h, more preferably from 200 to1000 kg/h, most preferably from 300 to 600 kg/h. The throughput per boreis preferably from 0.1 to 10 kg/h, more preferably from 0.5 to 5 kg/h,most preferably from 0.7 to 2 kg/h.

The start-up of the cocurrent spray dryer (5) can be done in thefollowing sequence:

-   -   starting the condenser column (12),    -   starting the ventilators (10) and (17),    -   starting the heat exchanger (20),    -   heating up the drying gas loop up to 95° C.,    -   starting the nitrogen feed via the nitrogen inlet (19),    -   waiting until the residual oxygen is below 4% by weight,    -   heating up the drying gas loop,    -   at a temperature of 105° C. starting the water feed (not shown)        and    -   at target temperature stopping the water feed and starting the        monomer feed via dropletizer unit (4)

The shut-down of the cocurrent spray dryer (5) can be done in thefollowing sequence:

-   -   stopping the monomer feed and starting the water feed (not        shown),    -   shut-down of the heat exchanger (20),    -   cooling the drying gas loop via heat exchanger (13),    -   at a temperature of 105° C. stopping the water feed,    -   at a temperature of 60° C. stopping the nitrogen feed via the        nitrogen inlet (19) and    -   feeding air into the drying gas loop (not shown)

To prevent damages the cocurrent spray dryer (5) must be heated up andcooled down very carefully. Any quick temperature change must beavoided.

The openings in the bottom of the internal fluidized bed may be arrangedin a way that the water-absorbent polymer particles flow in a cycle asshown in FIG. 7. The bottom shown in FIG. 7 comprises of four segments(62). The openings (63) in the segments (62) are in the shape of slitsthat guides the passing gas stream into the direction of the nextsegment (62). FIG. 8 shows an enlarged view of the openings (63).

The opening may have the shape of holes or slits. The diameter of theholes is preferred from 0.1 to 10 mm, more preferred from 0.2 to 5 mm,most preferred from 0.5 to 2 mm. The slits have a length of preferredfrom 1 to 100 mm, more preferred from 2 to 20 mm, most preferred from 5to 10 mm, and a width of preferred from 0.5 to 20 mm, more preferredfrom 1 to 10 mm, most preferred from 2 to 5 mm.

FIG. 9 and FIG. 10 show a rake stirrer (64) that may be used in theinternal fluidized bed. The prongs (65) of the rake have a staggeredarrangement. The speed of rake stirrer is preferably from 0.5 to 20 rpm,more preferably from 1 to 10 rpm most preferably from 2 to 5 rpm.

For start-up the internal fluidized bed may be filled with a layer ofwater-absorbent polymer particles, preferably 5 to 50 cm, morepreferably from 10 to 40 cm, most preferably from 15 to 30 cm.

Water-Absorbent Polymer Particles

The present invention further provides water-absorbent polymer particlesobtainable by the process according to the invention.

The inventive water-absorbent polymer particles have a mean sphericityfrom preferably from 0.80 to 0.95, more preferably from 0.82 to 0.93,even more preferably from 0.84 to 0.91, most preferably from 0.85 to0.90. The sphericity (SPHT) is defined as

${{SPHT} = \frac{4\pi \; A}{U^{2}}},$

where A is the cross-sectional area and U is the cross-sectionalcircumference of the polymer particles. The mean sphericity is thevolume-average sphericity.

The mean sphericity can be determined, for example, with the Camsizer®image analysis system (Retsch Technolgy GmbH; Haan; Germany):

For the measurement, the product is introduced through a funnel andconveyed to the falling shaft with a metering channel. While theparticles fall past a light wall, they are recorded selectively by acamera. The images recorded are evaluated by the software in accordancewith the parameters selected.

To characterize the roundness, the parameters designated as sphericityin the program are employed. The parameters reported are the meanvolume-weighted sphericities, the volume of the particles beingdetermined via the equivalent diameter xc_(min). To determine theequivalent diameter xc_(min), the longest chord diameter for a total of32 different spatial directions is measured in each case. The equivalentdiameter xc_(min) is the shortest of these 32 chord diameters. To recordthe particles, the so-called CCD-zoom camera (CAM-Z) is used. To controlthe metering channel, a surface coverage fraction in the detectionwindow of the camera (transmission) of 0.5% is predefined.

Water-absorbent polymer particles with relatively low sphericity areobtained by reverse suspension polymerization when the polymer beads areagglomerated during or after the polymerization.

The water-absorbent polymer particles prepared by customary solutionpolymerization (gel polymerization) are ground and classified afterdrying to obtain irregular polymer particles. The mean sphericity ofthese polymer particles is between approx. 0.72 and approx. 0.78.

The inventive water-absorbent polymer particles have a content ofhydrophobic solvent of preferably less than 0.005% by weight, morepreferably less than 0.002% by weight and most preferably less than0.001% by weight. The content of hydrophobic solvent can be determinedby gas chromatography, for example by means of the headspace technique.A hydrophobic solvent within the scope of the present invention iseither immiscible in water or only sparingly miscible. Typical examplesof hydrophobic solvents are pentane, hexane, cyclohexane, toluene.

Water-absorbent polymer particles which have been obtained by reversesuspension polymerization still comprise typically approx. 0.01% byweight of the hydrophobic solvent used as the reaction medium.

The inventive water-absorbent polymer have a dispersant content ofpreferably less than 0.5% by weight, more preferably less than 0.1% byweight and most preferably less than 0.05% by weight.

Water-absorbent polymer particles which have been obtained by reversesuspension polymerization still comprise typically at least 1% by weightof the dispersant, i.e. ethylcellulose, used to stabilize thesuspension.

The inventive water-absorbent polymer particles have a bulk densitypreferably from 0.6 to 1 g/cm³, more preferably from 0.65 to 0.9 g/cm³,most preferably from 0.68 to 0.8 g/cm³.

The average particle diameter (APD) of the inventive water-absorbentpolymer particles is preferably from 200 to 550 μm, more preferably from250 to 500 μm, most preferably from 350 to 450 μm.

The particle diameter distribution (PDD) of the inventivewater-absorbent polymer particles is preferably less than 0.7, morepreferably less than 0.65, more preferably less than 0.6.

The inventive water-absorbent polymer particles have a centrifugeretention capacity (CRC) of preferably from 35 to 100 g/g, morepreferably from 40 to 80 g/g, most preferably from 45 to 60 g/g.

The inventive water-absorbent polymer particles have a HC 60 value ofpreferably at least 80, more preferably of at least 85, most preferablyof at least 90.

The inventive water-absorbent polymer particles have an absorbency undera load of 21.0 g/cm² (AUL) of preferably from 15 to 60 g/g, morepreferably from 20 to 50 g/g, most preferably from 25 to 40 g/g.

The level of extractable constituents of the inventive water-absorbentpolymer particles is preferably from 0.1 to 30% by weight, morepreferably from 0.5 to 25% by weight, most preferably from 1 to 20% byweight.

The inventive water-absorbent polymer particles can be mixed with otherwater-absorbent polymer particles prepared by other processes, i.e.solution polymerization.

Fluid-Absorbent Articles

The present invention further provides fluid-absorbent articles. Thefluid-absorbent articles comprise of

-   -   (A) an upper liquid-pervious layer    -   (B) a lower liquid-impervious layer    -   (C) a fluid-absorbent core between (A) and (B) comprising        -   from 5 to 90% by weight fibrous material and from 10 to 95%            by weight water-absorbent polymer particles of the present            invention;        -   preferably from 20 to 80% by weight fibrous material and            from 20 to 80% by weight water-absorbent polymer particles            of the present invention;        -   more preferably from 30 to 75% by weight fibrous material            and from 25 to 70% by weight water-absorbent polymer            particles of the present invention;        -   most preferably from 40 to 70% by weight fibrous material            and from 30 to 60% by weight water-absorbent polymer            particles of the present invention;    -   (D) an optional acquisition-distribution layer between (A) and        (C), comprising        -   from 80 to 100% by weight fibrous material and from 0 to 20%            by weight water-absorbent polymer particles of the present            invention;        -   preferably from 85 to 99.9% by weight fibrous material and            from 0.01 to 15% by weight water-absorbent polymer particles            of the present invention;        -   more preferably from 90 to 99.5% by weight fibrous material            and from 0.5 to 10% by weight water-absorbent polymer            particles of the present invention;        -   most preferably from 95 to 99% by weight fibrous material            and from 1 to 5% by weight water-absorbent polymer particles            of the present invention;    -   (E) an optional tissue layer disposed immediately above and/or        below (C); and    -   (F) other optional components.

Fluid-absorbent articles are understood to mean, for example,incontinence pads and incontinence briefs for adults or diapers forbabies. Suitable fluid-absorbent articles including fluid-absorbentcompositions comprising fibrous materials and optionally water-absorbentpolymer particles to form fibrous webs or matrices for the substrates,layers, sheets and/or the fluid-absorbent core.

Suitable fluid-absorbent articles are composed of several layers whoseindividual elements must show preferably definite functional parametersuch as dryness for the upper liquid-pervious layer, vapor permeabilitywithout wetting through for the lower liquid-impervious layer, aflexible, vapor permeable and thin fluid-absorbent core, showing fastabsorption rates and being able to retain highest quantities of bodyfluids, and an acquisition-distribution layer between the upper layerand the core, acting as transport and distribution layer of thedischarged body fluids. These individual elements are combined such thatthe resultant fluid-absorbent article meets overall criteria such asflexibility, water vapor breathability, dryness, wearing comfort andprotection on the one side, and concerning liquid retention, rewet andprevention of wet through on the other side. The specific combination ofthese layers provides a fluid-absorbent article delivering both highprotection levels as well as high comfort to the consumer.

The products as obtained by the present invention are also very suitableto be incorporated into low-fluff, low-fiber, fluff-less, or fiber-lesshygiene article designs. Such designs and methods to make them are forexample described in the following publications and literature citedtherein and are expressly incorporated into the present invention: WO2010/133529 A2, WO 2011/084981 A1, US 2011/0162989, US 2011/0270204, WO2010/082373 A1, WO 2010/143635 A1, U.S. Pat. No. 6,972,011, WO2012/048879 A1, WO 2012/052173 A1 und WO 2012/052172 A1.

The present invention further provides fluid-absorbent articles,comprising water-absorbent polymer particles of the present inventionand less than 15% by weight fibrous material and/or adhesives in theabsorbent core.

The water-absorbent polymer particles and the fluid-absorbent articlesare tested by means of the test methods described below.

Methods:

The measurements should, unless stated otherwise, be carried out at anambient temperature of 23±2° C. and a relative atmospheric humidity of50±10%. The water-absorbent polymers are mixed thoroughly before themeasurement.

Free Swell Rate (FSR)

1.00 g (=W1) of the dry water-absorbent polymer particles is weighedinto a 25 ml glass beaker and is uniformly distributed on the base ofthe glass beaker. 20 ml of a 0.9% by weight sodium chloride solution arethen dispensed into a second glass beaker, the content of this beaker israpidly added to the first beaker and a stopwatch is started. As soon asthe last drop of salt solution is absorbed, confirmed by thedisappearance of the reflection on the liquid surface, the stopwatch isstopped. The exact amount of liquid poured from the second beaker andabsorbed by the polymer in the first beaker is accurately determined byweighing back the second beaker (=W2). The time needed for theabsorption, which was measured with the stopwatch, is denoted t. Thedisappearance of the last drop of liquid on the surface is defined astime t.

The free swell rate (FSR) is calculated as follows:

FSR[g/gs]=W2/(W1xt)

When the moisture content of the hydrogel-forming polymer is more than3% by weight, however, the weight W1 must be corrected for this moisturecontent.

Vortex

50.0±1.0 ml of 0.9% NaCl solution are added into a 100 ml beaker. Acylindrical stirrer bar (30×6 mm) is added and the saline solution isstirred on a stir plate at 60 rpm. 2.000±0.010 g of water-absorbentpolymer particles are added to the beaker as quickly as possible,starting a stop watch as addition begins. The stopwatch is stopped whenthe surface of the mixture becomes “still” that means the surface has noturbulence, and while the mixture may still turn, the entire surface ofparticles turns as a unit. The displayed time of the stopwatch isrecorded as Vortex time.

Residual Monomers

The level of residual monomers in the water-absorbent polymer particlesis determined by the EDANA recommended test method No. WSP 410.2-05“Residual Monomers”.

Particle Size Distribution

The particle size distribution of the water-absorbent polymer particlesis determined with the Camziser® image analysis sytem (Retsch TechnologyGmbH; Haan; Germany).

For determination of the average particle diameter and the particlediameter distribution the proportions of the particle fractions byvolume are plotted in cumulated form and the average particle diameteris determined graphically.

The average particle diameter (APD) here is the value of the mesh sizewhich gives rise to a cumulative 50% by weight.

The particle diameter distribution (PDD) is calculated as follows:

${{PDD} = \frac{x_{2} - x_{1}}{APD}},$

wherein x₁ is the value of the mesh size which gives rise to acumulative 90% by weight and x₂ is the value of the mesh size whichgives rise to a cumulative 10% by weight.

Mean Sphericity

The mean sphericity is determined with the Camziser® image analysissystem (Retsch Technology GmbH; Haan; Germany) using the particlediameter fraction from 100 to 1,000 μm.

Moisture Content

The moisture content of the water-absorbent polymer particles isdetermined by the EDANA recommended test method No. WSP 430.2-05“Moisture Content”.

Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity of the water-absorbent polymerparticles is determined by the EDANA recommended test method No. WSP441.2-05 “Centrifuge Retention Capacity”, wherein for higher values ofthe centrifuge retention capacity larger tea bags have to be used.

Absorbency Under No Load (AUNL)

The absorbency under no load of the water-absorbent polymer particles isdetermined analogously to the EDANA recommended test method No. WSP442.2-05 “Absorption Under Pressure”, except using a weight of 0.0 g/cm²instead of a weight of 21.0 g/cm².

Absorbency Under Load (AUL)

The absorbency under load of the water-absorbent polymer particles isdetermined by the EDANA recommended test method No. WSP 442.2-05“Absorption Under Pressure”.

Absorbency Under High Load (AUHL)

The absorbency under high load of the water-absorbent polymer particlesis determined analogously to the EDANA recommended test method No. WSP442.2-05 “Absorption Under Pressure”, except using a weight of 49.2g/cm² instead of a weight of 21.0 g/cm².

Bulk Density

The bulk density of the water-absorbent polymer particles is determinedby the EDANA recommended test method No. WSP 460.2-05 “Density”.

Extractables

The level of extractable constituents in the water-absorbent polymerparticles is determined by the EDANA recommended test method No. WSP470.2-05 “Extractables”.

Color Value (CIE Color Numbers [L, a, b])

Measurement of the color value is done by means of a colorimeter model“LabScan XE S/N LX17309” (HunterLab; Reston; U.S.A.) according to theCIELAB procedure (Hunterlab, Volume 8, 1996, Issue 7, pages 1 to 4).Colors are described by the coordinates L, a, and b of athree-dimensional system. L characterizes the brightness, whereby L=0 isblack and L=100 is white. The values for a and b describe the positionof the color on the color axis red/green resp. yellow/blue, wherebypositive a values stand for red colors, negative a values for greencolors, positive b values for yellow colors, and negative b values forblue colors. The HC60 value is calculated according to the formulaHC60=L-3b.

The measurement of the color value is in agreement with the tristimulusmethod according to DIN 5033-6.

The EDANA test methods are obtainable, for example, from the EDANA,Avenue Eugene Plasky 157, B-1030 Brussels, Belgium.

EXAMPLES

Preparation of the Base Polymer

Example 1

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m.

The drying gas was fed via a gas distributor (3) at the top of the spraydryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). Instead of the baghousefilter (9) any other filter and/or cyclone can be used. The drying gaswas nitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.79 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cyclindrical part as shown in FIG.3. Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 118° C. byadjusting the gas inlet temperature via the heat exchanger (20). The gasinlet temperature was 168° C. and the steam content of the drying gaswas 0.058 kg steam per kg dry gas.

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 120° C. and a steam content of 0.058 kg steam per kg drygas was fed to the internal fluidized bed (27) via line (25). The gasvelocity of the internal fluidized bed gas in the internal fluidized bed(27) was 0.65 m/s. The residence time of the product was 150 min. Thetemperature of the water-absorbent polymer particles in the internalfluidized bed was 80° C.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 45° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into sieve (29). The sieve (29) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit. A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 12.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.022% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.036 to 0.072% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochlorid, 0.0029% byweight of Bruggolite FF7, 0.054% by weight of sodiumperoxodisulfate andwater. The degree of neutralization was 71%. The feed per bore was 1.6kg/h.

The resulting water-absorbent polymer particles were analyzed. Theresults are summarized in Table 1.

TABLE 1 Effect of the azo initiator Azo Bulk Density CRC AUNL AULResidual Extractables Moisture FSR Example initiator [g/cm³] [g/g] [g/g][g/g] Monomers [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 1a 0.08*) 0.7432.6 37.5 24.2 15100 4.0 6.1 0.28 95.0 2.2 1.3 91.1 1b 0.12*) 0.73 36.644.8 24.8 13600 4.2 5.6 0.22 95.2 1.5 0.9 92.5 1c 0.16*) 0.76 47.0 55.125.1 11700 3.8 6.3 0.14 96.1 1.7 1.2 92.5 *)% by weight based on monomera)

Example 2

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m.

The drying gas was feed via a gas distributor (3) at the top of thespray dryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). Instead of the baghousefilter (9) any other filter and/or cyclone can be used. The drying gaswas nitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.62 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cyclindrical part as shown in FIG.3. Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 116° C. byadjusting the gas inlet temperature via the heat exchanger (20). The gasinlet temperature was 167° C. and the steam content of the drying gaswas 0.058 kg steam per kg dry gas.

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 123° C. and a steam content of 0.058 kg steam per kg drygas was fed to the internal fluidized bed (27) via line (25). The gasvelocity of the internal fluidized bed gas in the internal fluidized bed(27) was 0.65 m/s. The residence time of the product was 240 min. Thetemperature of the water-absorbent polymer particles in the internalfluidized bed was 84° C.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 45° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into sieve (29). The sieve (29) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit. A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 10.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.025% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0036% byweight of Bruggolite FF7, 0.054 to 0.270% by weight ofsodiumperoxodisulfate and water. The degree of neutralization was 71%.The feed per bore was 1.6 kg/h.

The resulting water-absorbent polymer particles were analyzed. Theresults are summarized in Table 2.

TABLE 2 Effect of the persulfate Residual Bulk Density CRC AUNL AULMonomers Extractables Moisture FSR Example Persulfate [g/cm³] [g/g][g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 2a 0.12*) 0.73 30.342.0 25.9 21400 4.7 5.5 0.28 94.9 2.0 1.4 90.7 2b 0.20*) 0.69 30.0 41.026.1 20100 4.8 5.5 0.28 94.3 2.3 1.8 88.9 2c 0.40*) 0.72 31.5 40.8 26.418100 4.8 4.5 0.29 94.2 2.1 3.0 85.2 2d 0.60*) 0.74 32.8 45.2 24.5 152004.9 4.8 0.30 94.5 1.6 3.7 83.4 *)% by weight based on monomer a)

Example 3

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m.

The drying gas was feed via a gas distributor (3) at the top of thespray dryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). Instead of the baghousefilter (9) any other filter and/or cyclone can be used. The drying gaswas nitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.56 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cyclindrical part as shown in FIG.3. Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 115° C. byadjusting the gas inlet temperature via the heat exchanger (20). The gasinlet temperature was 162° C. and the steam content of the drying gaswas 0.077 kg steam per kg dry gas.

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 114 to 184° C. and a steam content of 0.077 kg steam perkg dry gas was fed to the internal fluidized bed (27) via line (25). Thegas velocity of the internal fluidized bed gas in the internal fluidizedbed (27) was 0.65 m/s. The residence time of the product was 45 to 300min. The temperature of the water-absorbent polymer particles in theinternal fluidized bed was 71 to 116° C.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 50° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into sieve (29). The sieve (29) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit. A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 10.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.036% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.0043% by weight of Bruggolite FF7,0.270% by weight of sodiumperoxodisulfate and water. The degree ofneutralization was 71%. The feed per bore was 1.6 kg/h.

The resulting water-absorbent polymer particles were analyzed. Theresults are summarized in Table 3.

TABLE 3 Effect of temperature and residence time in the internalfluidized bed Bulk Residual Temperature Residence Time Density CRC AUNLAUL Monomers Extractables Moisture FSR Example [° C.] [min] [g/cm³][g/g] [g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 3a 71 45 0.8230.9 35.7 25.3 25200 6.3 12.3 0.23 93.4 2.7 1.2 89.8 3b 90 45 0.73 45.150.3 20.4 16300 7.2 8.9 0.21 93.6 1.2 2.9 84.9 3c 116 45 0.68 48.3 53.416.2 13500 9.3 4.6 0.23 92.6 0.6 4.3 79.7 3d 71 150 0.79 31.9 36.6 24.515000 4.1 10.2 0.21 93.4 2.5 1.7 88.3 3e 90 150 0.72 48.0 53.1 19.213000 5.2 5.3 0.24 93.7 1.4 3.1 84.4 3f 116 150 0.64 52.3 59.8 16.2 88006.1 1.1 0.26 94.4 0.2 4.8 80.0 3g 71 300 0.75 45.5 50.2 23.3 10500 3.67.2 0.21 93.8 2.1 1.8 88.4 3h 90 300 0.71 48.7 52.5 17.3 8100 4.5 4.40.24 93.6 1.1 4.8 79.2 3i 115 300 0.60 53.3 58.3 12.2 7200 5.6 1.0 0.2592.9 0.3 7.0 71.9 3j 75 300 0.73 46.2 51.2 21.3 9800 3.6 6.9 0.21 93.81.9 1.6 89.0 3l 75 240 0.72 45.9 51.8 18.9 10100 3.8 7.5 0.23 93.4 1.82.5 85.9 3k 77 300 0.70 46.9 52.9 19.7 9300 3.8 6.2 0.23 93.7 1.7 2.087.7

Example 4

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m.

The drying gas was feed via a gas distributor (3) at the top of thespray dryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). Instead of the baghousefilter (9) any other filter and/or cyclone can be used. The drying gaswas nitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.79 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cyclindrical part as shown in FIG.3. Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 116° C. byadjusting the gas inlet temperature via the heat exchanger (20). The gasinlet temperature was 169° C. and the steam content of the drying gaswas 0.058 kg steam per kg dry gas.

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 117° C. and a steam content of 0.058 to 0.225 kg steamper kg dry gas was fed to the internal fluidized bed (27) via line (25).The gas velocity of the internal fluidized bed gas in the internalfluidized bed (27) was 0.65 m/s. The residence time of the product was150 min. The temperature of the water-absorbent polymer particles in theinternal fluidized bed was 81° C.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 45° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into sieve (29). The sieve (29) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit. A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 10.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.036% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0029% byweight of Bruggolite FF7, 0.054% by weight of sodiumperoxodisulfate andwater. The degree of neutralization was 71%. The feed per bore was 1.6kg/h.

The resulting water-absorbent polymer particles were analyzed. Theresults are summarized in Table 4.

TABLE 4 Effect of the steam content in the internal fluidized bed SteamContent Bulk Density CRC AUNL AUL Residual Monomers ExtractablesMoisture FSR Example [kg/kg] [g/cm³] [g/g] [g/g] [g/g] [ppm] [wt. %][wt. %] [g/gs] L a b HC 60 4a 0.077 0.75 40.0 48.5 26.5 14000 4.2 5.70.27 96.1 1.0 2.9 87.4 4b 0.172 0.70 36.6 44.7 25.1 9600 5.8 7.8 0.1896.1 0.9 3.0 87.1 4c 0.225 0.71 37.4 45.0 25.4 1400 2.5 10.7 0.08 94.22.6 1.7 89.1

1. (canceled)
 2. Polymer particles according to claim 14, wherein the heated gas phase is flowing cocurrent to the droplets through the reaction zone.
 3. Polymer particles according to claim 14, wherein the monomer solution comprises from 0.05 to 0.15% by weight, based on monomer a), of the at least one persulfate.
 4. Polymer particles process according to claim 14, wherein the monomer solution comprises from 0.2 to 0.5% by weight, based on monomer a), of the at least one azo initiator.
 5. Polymer particles according to claim 14, wherein the temperature of the gas leaving the reaction zone is from 110 to 120° C.
 6. Polymer particles according to claim 14, wherein the temperature of the gas entering the reaction zone is from 160 to 200° C.
 7. Polymer particles according to claim 14, wherein a gas velocity inside the reaction zone is from 0.1 to 2.5 m/s.
 8. Polymer particles according to claim 14, wherein the temperature of the water-absorbent polymer particles during the thermal posttreatment is from 60 to 80° C.
 9. Polymer particles according to claim 14, wherein the residence time of the water-absorbent polymer particles in the fluidized bed is from 120 to 240 minutes.
 10. Polymer particles according to claim 14, wherein a gas velocity inside the fluidized bed is from 0.3 to 2.5 m/s.
 11. Polymer particles according to claim 14, wherein the gas entering the fluidized bed comprises from 0.02 to 0.15 kg steam per kg of dry gas.
 12. Polymer particles according to claim 14, wherein the ethylenically unsaturated monomer which bears an acid group is an ethylenically unsaturated carboxylic acid.
 13. Polymer particles according to claim 14, wherein the ethylenically unsaturated monomer which bears an acid group is acrylic acid.
 14. Water-absorbent polymer particles prepared according to by polymerizing droplets of a) at least one ethylenically unsaturated monomer which bears an acid group and optionally is at least partly neutralized, b) at least one crosslinker, c₁) from 0.01 to less than 0.3% by weight, based on monomer a), of at least one persulfate, c₂) at least 0.05% by weight, based on monomer a), of at least one azo initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), e) optionally one or more water-soluble polymer, and f) water, in a surrounding heated gas phase in a reaction zone and thermal posttreatment in a fluidized bed, wherein a temperature of a gas leaving the reaction zone is less than 150° C., a temperature of the water-absorbent polymer particles during the thermal posttreatment is less than 100° C., and a residence time of the water-absorbent polymer particles in the fluidized bed is from 60 to 300 minutes.
 15. Water-absorbent polymer particles having a bulk density of at least 0.65 g/cm³, a centrifuge retention capacity of at least 35 g/g, and a HC 60 value of at least
 80. 16. Polymer particles according to claim 15, wherein the polymer particles have a bulk density of at least 0.7 g/cm³.
 17. Polymer particles according to claim 15, wherein the polymer particles have a centrifuge retention capacity of at least 40 g/g.
 18. Polymer particles according to claim 15 wherein the polymer particles have a HC 60 value of at least
 90. 19. A fluid-absorbent article, comprising (A) an upper liquid-pervious layer, (B) a lower liquid-impervious layer and (C) a fluid-absorbent core between the layer (A) and the layer (B), comprising from 5 to 90% by weight fibrous material and from 10 to 95% by weight water-absorbent polymer particles according to claim 14, (D) an optional acquisition-distribution layer between (A) and (C), comprising from 80 to 100% by weight fibrous material and from 0 to 20% by weight water-absorbent polymer particles according to claim 14, (E) an optional tissue layer disposed immediately above and/or below (C); and (F) other optional components.
 20. A fluid-absorbent article, comprising water-absorbent polymer particles according to claim 14 and less than 15% by weight fibrous material and/or adhesives in the absorbent core.
 21. A fluid-absorbent article, comprising (A) an upper liquid-pervious layer, (B) a lower liquid-impervious layer and (C) a fluid-absorbent core between the layer (A) and the layer (B), comprising from 5 to 90% by weight fibrous material and from 10 to 95% by weight water-absorbent polymer particles according to claim 15, (D) an optional acquisition-distribution layer between (A) and (C), comprising from 80 to 100% by weight fibrous material and from 0 to 20% by weight water-absorbent polymer particles according to claim 15, (E) an optional tissue layer disposed immediately above and/or below (C); and (F) other optional components.
 22. Polymer particles according to claim 14, wherein the monomer solution comprises from 0.1 to 0.15% by weight, based on monomer a) of the persulfate. 